In the field of power conversion, the translation of a d-c voltage into an a-c voltage is generally accomplished by a controllable switching system termed an inverter. Different types of inverters have been developed and implemented. A bridge type inverter generally comprises two thyristors or other semiconductor switches coupled in series between the bus conductors, and the a-c load conductor is coupled to the common connection between the thyristors. A pair of commutating capacitors is coupled between the bus conductors, with a common connection at their electrical midpoint coupled to the center of a commutating conductor, which is itself coupled between the thyristors. In this brige inverter arrangement, different systems have been employed to be certain the commutating capacitors are sufficiently charged before the d-c voltage is applied to the bus conductors. Some have devised specific arrangements of the thyristor circuit to facilitate the pre-charge operation. See for example U.S. Pat. No. 3,559,038, entitled "Inverter with Pre-charge Circuit" which issued to Stanley Krauthmer on Jan. 26, 1971.
The auxiliary-commutated (aux-comm) inverter, in lieu of a passive commutating arrangement, ultilizes two additional switches which are selectively gated on to shut off the main thyristors. A similar problem has been encountered in the aux-comm inverter art, as it is still necessary to initially charge the commutating capacitor to be certain that the inverter will operate effectively during system start-up. Sometimes this problem has been approached by a brute-force method, in which a relay is energized when the system is initially powered up, to complete a circuit for pre-charging the commutating capacitor, while at the same time some other means is utilized to delay application of the d-c energizing voltage to the inverter bus. Of course this entails additional expense for the relay and control components, and occupies a larger physical space than would otherwise be required.
It is therefore a main object of the present invention to provide an improved start-up energization arrangement for an aux-comm inverter, which is simpler and less expensive than known arrangements.